Centrifuges are essential instruments in any biological or chemical laboratory as they allow separation of a sample into different components based on each component's density. A typical centrifuge consists of a rotor encased in a housing. The rotor is powered by a drive motor or some other force that allows it to complete a set number of rotations or revolutions per minute (rpm). Attached to the rotor are holders in which to place sample containers, such as test tubes or well plates. These holders are placed symmetrically around the circumference of the rotor. The sample containers are balanced to ensure a symmetric mass distribution around the rotor. The sample containers are placed in the holders and each sample may then be spun and separated into various components or fractions.
Separation of the samples occurs because each component has a different density and thus a different sedimentation velocity. Sedimentation velocity is a measure of how fast a component will migrate through other more buoyant sample components as a result of the centrifugal field generated by the centrifuge.
Using centrifugation, a variety of samples can each be separated into various components. For example, specific cell organelles can be isolated, particles can be removed from a suspension, and a mixture of liquids of different density can be separated. In general, the degree of separation of components within a given sample is determined by the magnitude of the centrifugal force applied to the sample and the length of time for which the sample is spun. In turn, the magnitude of the centrifugal force is a function of the nature of the rotor used to hold the sample containers and the speed of rotation (number of rpm) of the rotor.
Centrifuges are typically fairly bulky, rectangular instruments that are positioned on the floor or on a table. They are usually able to accommodate only one type of sample container, such as a test tube or a multi-well plate (also known as a microtiter plate). The type of sample container determines the size of the centrifuge housing. For example, centrifuges for well plates are relatively large because the well plates require a lot of room during spinning. The number of samples that can be spun at one time is usually limited by size and space constraints. In addition, much time is needed to spin down samples due to large drift distance (see definition of drift distance herein below under DETAILED DESCRIPTION). Laboratory protocols that use a large number of samples normally require a lot of time for centrifugation. Lastly, before centrifugation, the sample containers must be balanced in terms of their mass and placed symmetrically around the rotor. If the rotor is unbalanced, breakage of the centrifuge can result, and the sample to be separated may be lost. Tasks associated with centrifugation are usually performed manually, although in some cases robotic arms may be available. Unfortunately, robotic arms are very expensive and require a custom designed centrifuge housing to accommodate their use.
Each centrifuge has a maximum rpm it can reach. The maximum rpm is determined by the strength of the drive motor, the mechanical strength of the rotor, and the mechanical strength of the sample containers. Low speed centrifuges, such as Beckman's KneeWell Centrifuge, can reach up to 10,000 rpm, while high speed centrifuges, such as DuPont's Sorval High Speed Centrifuge can reach up to 20,000 rpm. The rpm and rotor size used determine the centrifugal field generated, which in turn affects the sedimentation velocity of the sample components. For a given rotor, higher rpm increases the centrifugal field and the sedimentation velocity. Thus, for a given size rotor, a higher rpm decreases the amount of time necessary to spin down or separate a sample. Centrifuges often come equipped with a timer to allow automatic stoppage of rotor rotation after a set period of time.
The main limitations of centrifuges are the need for a large amount of manual labor to load and unload them, the small number of samples that can be spun down at one time, and the length of time it takes to spin down samples. In addition, the maximum acceleration used for prior art centrifuges may be limited by the mechanical strength of the sample containers, thereby increasing the amount of time needed to spin down samples. This is particularly true in the case of spinning multi-well plates using prior art systems and methods. Although at least some of these problems could be overcome by the use of robotic arms and the purchase of more centrifuges, the cost and space requirements would be prohibitive for most laboratories.